error C2678: binary '=' : no operator found which takes a left-hand operand of type 'const std::string' (or there is no acceptable conversion)

Question

I am really confused on why I am getting following compilation error. Microsoft Visual Studio Compiler.

error C2678: binary '=' : no operator found which takes a left-hand operand of type 'const std::string' (or there is no acceptable conversion)

#include <stdio.h>
#include <iostream>
#include <sstream>
#include <iterator>

class MyException {
public:
    MyException(    std::string message, 
                        int line = 0) : m_message(message),
                                        m_line(line) {}
    const char* what() const throw(){
        if ( m_line != 0 ) {
            std::ostringstream custom_message;
            custom_message << "Parsing Error occured at ";
            custom_message << m_line << " Line : ";
            custom_message << m_message;        
            m_message = custom_message.str();
        }
        return m_message.c_str();
    }
private:
    std::string m_message;
    int m_line;
};
int main(int argc, char **argv) {
    try {
        // do something
    }catch(MyException &e){
        std::cout << e.what();
    }
}

Error is coming at line m_message = custom_message.str();

Answer 1

You declare the method as const

const char* what() const throw(){

but then you try to change the object

m_message = custom_message.str();

so you get an error.

What you should do instead is construct the custom message in the constructor.

class MyException {
public:
    MyException(const std::string& message, int line = 0) : 
        m_message(message), m_line(line) {
        if ( m_line != 0 ) {
            std::ostringstream custom_message;
            custom_message << "Parsing Error occured at ";
            custom_message << m_line << " Line : ";
            custom_message << m_message;        
            m_message = custom_message.str();
        }
    }
    const char* what() const throw(){
        return m_message.c_str();
    }
private:
    std::string m_message;
    int m_line;
};

Also I changed your code to pass the std::string by reference, which is usual practice.

Answer 2

You are trying to assign to MyException::m_message inside a const-qualified method MyException::what(). Inside such what() the entire *this object is considered to be const, which means that m_message member is also const. You can't assign anything to a const-qualified std::string object, since std::string's assignment operator requires a modifiable (i.e. a non-const one) object on the left-hand side. You are supplying a const one.

If you really want to be able to modify the m_message inside what(), you should declare it as mutable member of the class (in this case it appears to be a good idea). Or use some other approach.

As @john noted, in your specific case it makes more sense to actually build m_message in constructor instead of postponing it till what(). I don't really understand why you'd even want to rebuild your m_message every time you call what(). Unless your m_line is expected to change somehow from one call to what() to another, there's really no need to do it every time.